I made the first documented single speed rear hub motor here. Very light weight, very simple, and fast - total bike weight is 29lbs with a battery good for about an hour. Simple: My handle bars just have the two brakes and a subtle throttle button.

It works great for me because my routes are flat to rolling hills. The trick with a hub motor is to not spend much time at more than 1/2 its top speed. Full throttle for too long at lower speeds is going to generate too much heat.

If you are riding in the flats, you don't really need more than one gear. And light weight is a lot of fun. But, you aren't going to save a ton of weight by removing the gears.